On the origin of the ''core-free'' morphology in microinjection-molded HDPE

This study investigates the morphology of a high-density polyethylene processed with microinjection molding. Previous work pointed out that a "core-free" morphology exists for a micropart (150-µm thick), contrasting with the well-known "skin-core" morphology of a conventional part (1.5-mm thick). Local analyses are now conducted in every structural layer of these samples. Transmission electron microscopy observations reveal highly oriented crystalline lamellae perpendicular to the flow direction in the micropart. Image analysis also shows that lamellae are thinner. Wide-angle X-ray diffraction measurements using a microfocused beam highlight that highly oriented shish–kebab morphologies are found through the micropart thickness, with corresponding orientation function close to 0.8. For the macropart, quiescent crystallized morphologies are found with few oriented structures. Finally, the morphology within the micropart is more homogeneous, but the crystalline structures created are disturbed due to the combined effects of flow-induced crystallization and thermal crystallization during processing.